1. Field of the Invention
The present invention is generally related to optical transport systems. More particularly, aspects of the present invention are directed to transportation of multiple asynchronous data streams over an optical transport system using high order modulation.
2. Description of Related Art
A number of existing networking scenarios involve the transporting of a number of data streams from routers or switches over various geographical areas, such as across a metropolitan area or a regional network, in a manner that optimizes the uses of available optical fiber spectrum. The incoming data streams are typically asynchronous, typically out of phase and with clock rates within +/−100 ppm, as they may arrive from different, independent sources.
Some solutions employ time division multiplexing (“TDM”), for instance using a Synchronous Optical NETwork (“SONET”)/Synchronous Digital Hierarchy (“SDH”) or Optical Transport Network (“OTN”) hierarchy in which the lower speed services are synchronously multiplexed to a higher data rate for transmission. Other solutions employ Wavelength Division Multiplexing (“WDM”), where different incoming data streams are mapped to different wavelengths and then multiplexed on to a single fiber.
In TDM, the incoming asynchronous data streams are mapped to transport containers which are then TDM multiplexed to create a higher speed container. This higher speed container (stream) is then transmitted using a variety of optical modulation techniques ranging from simple on-off keying to high-order modulation based on phase and polarization. However, TDM is limited by the fastest electronic transmitting, receiving, channel impairment and processing technology available. Therefore, important deficiencies of TDM-only transport include the cost, complexity and power consumption burdens of the TDM multiplexing stage and subsequent de-serialization. In general, the TDM aggregation approach may introduce additional multiplexing/demultiplexing complexity as well as the cost premium of higher speed optics and electronics.
On the other hand, if conventional WDM technology (including Dense WDM “DWDM”) is used to transport these lower-speed asynchronous signals, the spectral efficiency is very low. Hence, the full transmission capacity of the fiber may not be utilized.